meson: deduplicate access to SHA1/SHA256 backend options

We've got a couple of repeated calls to `get_option()` for the SHA1 and
SHA256 backend options. While not an issue, it makes the code needlessly
verbose.

Fix this by consistently using a local variable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

meson.build

@@ -1326,6 +1326,8 @@ if not meson.is_cross_build() and fs.exists('/dev/tty')
 endif
 
 https_backend = get_option('https_backend')
+sha1_backend = get_option('sha1_backend')
+sha256_backend = get_option('sha256_backend')
 
 security_framework = dependency('Security', required: https_backend == 'CommonCrypto')
 core_foundation_framework = dependency('CoreFoundation', required: security_framework.found())
@@ -1333,7 +1335,7 @@ if https_backend == 'auto' and security_framework.found()
   https_backend = 'CommonCrypto'
 endif
 
-openssl_required = https_backend == 'openssl' or get_option('sha1_backend') == 'openssl' or get_option('sha256_backend') == 'openssl'
+openssl_required = https_backend == 'openssl' or sha1_backend == 'openssl' or sha256_backend == 'openssl'
 openssl = dependency('openssl', required: openssl_required, default_options: ['default_library=static'])
 if https_backend == 'auto' and openssl.found()
   https_backend = 'openssl'
@@ -1354,7 +1356,6 @@ if https_backend != 'openssl'
   libgit_c_args += '-DNO_OPENSSL'
 endif
 
-sha1_backend = get_option('sha1_backend')
 if sha1_backend == 'sha1dc'
   libgit_c_args += '-DSHA1_DC'
   libgit_c_args += '-DSHA1DC_NO_STANDARD_INCLUDES=1'
@@ -1382,7 +1383,6 @@ else
   error('Unhandled SHA1 backend ' + sha1_backend)
 endif
 
-sha256_backend = get_option('sha256_backend')
 if sha256_backend == 'openssl'
   libgit_c_args += '-DSHA256_OPENSSL'
   libgit_dependencies += openssl